The subject matter disclosed herein relates generally to an electric motor having an asymmetric design and, more specifically, to a permanent magnet (PM) machine having a physical construction such that at least one of the rotor and the stator has an asymmetric design.
PM motors include a set of magnets in the rotor. The magnets are either inserted in slots within the rotor (i.e., interior permanent magnets) or mounted to the outer surface of the rotor (i.e., surface permanent magnets). The magnets are configured such that the magnetic field varies in strength at different locations around the rotor. The magnets may be, for example, shaped such that more magnetic material exists in a central area of the magnet and less magnetic material exists toward the edge of the magnet or inserted within the rotor such that a portion of the magnet is further from the surface of the rotor. The magnetic field produced by the permanent magnets interacts with the field generated by a stator current to control rotation of the motor.
In a PM motor, various numbers of slots and windings are present in the stator and various numbers of poles are present in the rotor. The slots and windings may belong to a single phase or may be divided among multiple phases (e.g., 3 phases). In a multiple phase PM motor, one winding for each phase is wound around successive teeth establishing one set of phase windings. Additional sets of phase windings are then repeated around the stator. The poles typically alternate between north and south poles around the rotor. A single electrical cycle applied to the stator causes the rotor to rotate an angular distance equivalent to the span of one set of phase windings.
Typically, it is desirable to manufacture PM motors with a symmetric construction. With a symmetric construction, each set of phase windings is identical and each of the north or south poles in the rotor is identical. As a result of the symmetry, the PM motor exhibits uniform performance as the rotor passes each set of phase windings. Historically, symmetry has been desirable to provide uniform operation, or at least periodic operation, as each pole pair passes one set of phase windings, through one revolution of the PM motor. This uniform or periodic operation helps provide uniform wear of moving parts, bearing elements, and the like within the PM motor.
However, this uniform performance through one revolution of the PM motor is not without certain drawbacks. Numerous techniques for determining angular position of the rotor without use of a position sensor have been developed. However, because performance of the motor is periodic as the rotor passes each set of phase windings, the angular position may only be determined with respect to one set of phase windings. These sensorless techniques for determining angular position do not provide absolute mechanical position of the rotor. If, for example, the stator has nine windings, the rotor may be in any one of three different positions.
It is also known that a certain amount of torque ripple may be present in a PM motor. Although the poles are constructed such that the magnetic field varies in strength across the angular span of the pole, the variation is not sinusoidal and may include a torque ripple, or periodic variation, across the span of each pole. Although symmetric construction of the PM motor may result in uniform wear and uniform or periodic operation of the PM motor, symmetric construction may result in a greater amplitude of torque ripple than desired.
Thus, it would be desirable to provide a PM motor having one or more asymmetries to improve one or more operating characteristics of the PM motor.